The present disclosure relates to a symmetric hybrid supercapacitor containing LiMnxFe1-xPO4. The disclosure also relates to the use of LiMnxFe1-xPO4 as electrode material for a hybrid supercapacitor. The condition applies that 0.1<x<0.9.
Hybrid supercapacitors (HSCs), for example lithium ion capacitors, are a new generation of supercapacitors which can provide more power than lithium ion batteries. Although lithium ion batteries have a high energy density of more than 100 Wh/kg, they are able to release this energy only slowly. Hybrid supercapacitors have a higher energy density than supercapacitors (EDLCs/SCs), which are able to provide a power release of more than 100 kW/kg but have only a low energy density. Hybrid supercapacitors can be charged, for example, by means of short high-energy pulses as occur in the braking energy recuperation of motor vehicles. The electrical energy recovered in this way can subsequently be used to accelerate the motor vehicle. This enables saving of fuel and the reduction of carbon dioxide emissions. Hybrid supercapacitors are also being considered for use as an energy source in power tools. Since hybrid supercapacitors are a new technology compared to conventional supercapacitors and lithium ion batteries, only a few products are commercially available to date. Usually, in fields of application that would be suitable for hybrid supercapacitors, oversized lithium ion batteries are used, which, because of their size, are capable of providing the power required for the application in question.
Hybrid supercapacitors can be divided into two different categories according to the cell construction: symmetric and asymmetric hybrid supercapacitors. Asymmetric hybrid supercapacitors have an electrode, the material of which stores energy through reversible faradaic reaction. This may be a hybridized electrode. The second electrode is purely capacitative, meaning that it stores energy via the construction of a Helmholtz double layer. This construction is in common use particularly for first-generation hybrid supercapacitors, since it has an electrode configuration corresponding to the construction of lithium ion battery electrodes or supercapacitor electrodes, and so it is possible to utilize known electrode production methods. Lithium ion capacitors are one example of an asymmetric hybrid supercapacitor. Lithiated graphite or another form of a lithiatable carbon is used therein as anode. This enables a maximum voltage window of up to 4.3 V. However, SEI (solid electrolyte interface) formation at the anode is unavoidable in the case of use of anode materials having an intercalation potential close to 0 V vs. Li/Li+, for example graphite. This is typically countered by specific cell modification, for example by electrolyte additives such as vinylene carbonate, in order to stabilize the SEI layer and prevent further electrolyte breakdown. The second type is symmetric hybrid supercapacitors consisting of two internally hybridized electrodes having both faradaic and capacitatively active materials. Through this combination, it is possible to considerably increase the power density of the hybrid supercapacitors compared to conventional lithium ion batteries or the energy density compared to conventional supercapacitors. In addition, it is possible to utilize synergistic effects between the two active electrode materials in the two electrodes. Carbon as an electrode constituent additionally enables faster provision of energy from the two electrodes, since it improves the electrical conductivity of the electrodes. High-porosity carbon can also function as a shock absorber for high currents. Symmetric hybrid supercapacitors are superior to asymmetric hybrid supercapacitors in pulsed operation.
The cathode of symmetric hybrid supercapacitors may contain LiMn2O4 or LiFePO4. LiMn2O4 has a spinel structure and has a good voltage profile to fill the window of electrolyte stability with an intercalation plateau between 3.8 and 4.2 V versus Li/Li+. Moreover, LiMn2O4 enables three-dimensional diffusion of lithium ions, which enables rapid charging and discharging of the hybrid supercapacitor. However, there is dissolution of manganese(II) cations over the lifetime, which limits the lifetime of the catalyst. Moreover, the spinel structure can be damaged by Jahn-Teller distortions at high charge depths. LiFePO4 has an olivine structure. It can be produced from readily available and environmentally friendly materials and is known to be the safest cathode material in common use for lithium ion batteries. For a faradaic intercalation material, it additionally has a long lifetime. However, its intercalation plateau is 3.45 V versus Li/Li+. Therefore, with LiFePO4 as cathode material, it is not possible to fully exhaust the available voltage window of hybrid supercapacitors, and so it is not possible to achieve the maximum energy density possible for this capacitor type. Its low ion conductivity additionally limits the charging and discharging rate of the hybrid supercapacitor.